fix(vertex): floor reasoning_effort to low on gemini-3.7-flash

gemini-3.7-flash dropped THINKING_LEVEL_MINIMAL support on Vertex; only
LOW, MEDIUM, and HIGH are accepted. reasoning_effort values minimal,
none, and disable previously mapped to "minimal" via the broad
gemini-3.x-flash substring match and returned HTTP 400. They now floor
to "low" so they resolve to a level Vertex accepts.

Fixes #37314

litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py

@@ -272,6 +272,21 @@ class VertexGeminiConfig(VertexAIBaseConfig, BaseConfig):
             return True
         return False
 
+    @staticmethod
+    def _is_gemini3_flash_supports_minimal(model: str) -> bool:
+        """
+        Whether a gemini-3.x-flash model accepts THINKING_LEVEL_MINIMAL.
+
+        gemini-3.7-flash dropped MINIMAL support: Vertex only accepts LOW, MEDIUM, and
+        HIGH for it, while earlier 3.x-flash models (3.0, 3.1, 3.5, 3.6) still accept it.
+        """
+        lowered: Final = model.lower()
+        return (
+            "flash" in lowered
+            and "gemini-3" in lowered
+            and "gemini-3.7" not in lowered
+        )
+
     @staticmethod
     def _forward_gemini_function_call_id(model: str) -> bool:
         """
@@ -856,10 +871,8 @@ class VertexGeminiConfig(VertexAIBaseConfig, BaseConfig):
         Returns:
             GeminiThinkingConfig with thinkingLevel and includeThoughts
         """
-        # Check if this is gemini-3-flash which supports MINIMAL thinking level
-        # Covers gemini-3-flash, gemini-3-flash-preview, gemini-3.1-flash, gemini-3.1-flash-lite-preview,
-        # gemini-3.5-flash, and any future 3.x-flash variants.
-        is_gemini3flash: Final = model and ("flash" in model.lower() and "gemini-3" in model.lower())
+        is_gemini3flash: Final = bool(model) and VertexGeminiConfig._is_gemini3_flash_supports_minimal(model)
+        is_gemini3_flash_family: Final = bool(model) and "flash" in model.lower() and "gemini-3" in model.lower()
         is_gemini31pro: Final = model and ("gemini-3.1-pro-preview" in model.lower())
         if reasoning_effort == "minimal":
             if is_gemini3flash:
@@ -869,20 +882,20 @@ class VertexGeminiConfig(VertexAIBaseConfig, BaseConfig):
         elif reasoning_effort == "low":
             return {"thinkingLevel": "low", "includeThoughts": True}
         elif reasoning_effort == "medium":
-            if is_gemini31pro or is_gemini3flash:
+            if is_gemini31pro or is_gemini3_flash_family:
                 return {"thinkingLevel": "medium", "includeThoughts": True}
             else:
                 return {"thinkingLevel": "high", "includeThoughts": True}
         elif reasoning_effort == "high":
             return {"thinkingLevel": "high", "includeThoughts": True}
         elif reasoning_effort == "disable":
-            # Gemini 3 cannot fully disable thinking, so we use "minimal" for gemini-3-flash-preview, "low" for others
+            # Gemini 3 cannot fully disable thinking, so we use "minimal" for the models that accept it, "low" for others.
             if is_gemini3flash:
                 return {"thinkingLevel": "minimal", "includeThoughts": False}
             else:
                 return {"thinkingLevel": "low", "includeThoughts": False}
         elif reasoning_effort == "none":
-            # For gemini-3-flash-preview, use "minimal" instead of "low"
+            # Same as disable: "minimal" where accepted, "low" otherwise.
             if is_gemini3flash:
                 return {"thinkingLevel": "minimal", "includeThoughts": False}
             else:
@@ -953,8 +966,11 @@ class VertexGeminiConfig(VertexAIBaseConfig, BaseConfig):
                     params["includeThoughts"] = True
                     # Follow provider defaults unless explicitly opted into legacy behavior.
                     if litellm.enable_gemini_default_thinking_level_low is True:
-                        is_gemini3flash: Final = "gemini-3" in model.lower() and "flash" in model.lower()
-                        params["thinkingLevel"] = "minimal" if is_gemini3flash else "low"
+                        params["thinkingLevel"] = (
+                            "minimal"
+                            if VertexGeminiConfig._is_gemini3_flash_supports_minimal(model)
+                            else "low"
+                        )
             else:
                 # Thinking disabled
                 params["includeThoughts"] = False

tests/llm_translation/test_gemini.py

@@ -1806,6 +1806,64 @@ def test_gemini_31_flash_lite_reasoning_effort_minimal():
     ), "gemini-3.1-flash-lite-preview should use thinkingLevel, not thinkingBudget"
 
 
+def test_gemini_37_flash_reasoning_effort_no_minimal():
+    """
+    gemini-3.7-flash dropped THINKING_LEVEL_MINIMAL support on Vertex; only LOW, MEDIUM,
+    and HIGH are accepted. reasoning_effort values that previously mapped to "minimal"
+    (minimal, none, disable) must resolve to "low" instead so they return 200.
+
+    Regression test for: https://github.com/BerriAI/litellm/issues/37314
+    """
+    from litellm.llms.vertex_ai.gemini.vertex_and_google_ai_studio_gemini import (
+        VertexGeminiConfig,
+    )
+
+    # Values that used to map to "minimal" must now floor at "low" on 3.7.
+    for reasoning_effort in ("minimal", "none", "disable"):
+        result = VertexGeminiConfig._map_reasoning_effort_to_thinking_level(
+            reasoning_effort=reasoning_effort,
+            model="gemini-3.7-flash",
+        )
+        assert (
+            result["thinkingLevel"] == "low"
+        ), f"gemini-3.7-flash {reasoning_effort} should map to 'low', got '{result['thinkingLevel']}'"
+        assert result["includeThoughts"] is (reasoning_effort != "none" and reasoning_effort != "disable")
+
+    # Explicit low/medium/high are unaffected.
+    for reasoning_effort, expected_level in (
+        ("low", "low"),
+        ("medium", "medium"),
+        ("high", "high"),
+    ):
+        result = VertexGeminiConfig._map_reasoning_effort_to_thinking_level(
+            reasoning_effort=reasoning_effort,
+            model="gemini-3.7-flash",
+        )
+        assert (
+            result["thinkingLevel"] == expected_level
+        ), f"gemini-3.7-flash {reasoning_effort} should map to '{expected_level}', got '{result['thinkingLevel']}'"
+
+
+def test_gemini_36_flash_reasoning_effort_minimal_still_supported():
+    """
+    gemini-3.6-flash still accepts THINKING_LEVEL_MINIMAL, so the fix for 3.7 must not
+    regress the earlier 3.x-flash models.
+    """
+    from litellm.llms.vertex_ai.gemini.vertex_and_google_ai_studio_gemini import (
+        VertexGeminiConfig,
+    )
+
+    for model in ("gemini-3.6-flash", "gemini-3.5-flash"):
+        result = VertexGeminiConfig._map_reasoning_effort_to_thinking_level(
+            reasoning_effort="minimal",
+            model=model,
+        )
+        assert (
+            result["thinkingLevel"] == "minimal"
+        ), f"{model} should keep thinkingLevel='minimal', got '{result['thinkingLevel']}'"
+        assert result["includeThoughts"] is True
+
+
 def test_gemini_image_size_limit_exceeded(monkeypatch):
     """
     Test that large images exceeding MAX_IMAGE_URL_DOWNLOAD_SIZE_MB are rejected.